1. Field of the Invention
The invention relates generally to a method and apparatus for making semiconductor devices using lithography. More particularly, the invention relates to a gas-filled enclosure between a mask protective device and a patterned mask, and a method and apparatus for removing a gas from the enclosure and adding a different gas.
2. Background Information
Photolithography is a process frequently used in processes to manufacture semiconductor devices. During photolithography, a light-sensitive layer on a semiconductor device is selectively exposed to light through the use of a reticle or mask. Light is transmitted toward the light-sensitive layer, through the reticle, which contains transparent regions that transmit light to the light-sensitive layer and opaque regions that prevent exposure of certain areas of the light-sensitive layer to the light. Typically, the reticle is a transparent quartz plate with a pattern defined by opaque chrome included on one side of the quartz plate. The transparent and opaque regions are associated with circuitry to be created on the semiconductor device. The exposed portions of the light-sensitive layer are transformed, allowing them to be removed by solvents, to create the circuitry of the semiconductor device.
FIG. 1 shows a prior art pellicle-reticle system 100. The pellicle-reticle system 100 includes a reticle 110 protected from ambient particles, by a pellicle 120. The pellicle 120 is typically a light-transparent polymeric film attached to a rigid frame. Particles, such as airborne particles, settle on the pellicle 120, rather than on the reticle 110. The pellicle 120 is typically separated from the reticle 110 by a short distance sufficient to eliminate or dissipate effects of particles on the surface of the pellicle 120 from creating a shadow on the reticle 110 or on a semiconductor device. Without the pellicle 120, such particles could create unintended images on the semiconductor device and alter the circuitry. Accordingly, the pellicle 120 allows particles to be collected a short distance away from the reticle 110, where they will be out of focus on the wafer surface, and will not generate circuitry defects.
The pellicle 120 is typically connected to the reticle 110 by a wall 130. The pellicle 120, the reticle 110, and the wall 130 create an enclosed volume 140, typically of air. A single, small pressure equalization orifice 150 is typically provided to equalize the pressure across the pellicle 120. This orifice 150 prevents changes in the pressure of the enclosed volume, or ambient air from damaging or altering optical properties of the pellicle 120. The orifice 150 is made small to discourage external particles from entering the enclosed volume 140 and surfacing directly on the reticle 110. For example, considering an enclosed volume approximately 115 mm in length and 90 mm in width, the single, small orifice 150 is typically less than approximately 3 mm in length by 1 mm in height. For the same reason, the orifice 150 is sometimes a convoluted passageway in order to trap particles in the passageway before entering the enclosed volume. Multiple orifices are not used, since multiple orifices promote convective flow into and out of the enclosure, which is not desired. The primary purpose of the pellicle 120 is to keep particles off of the surface of the reticle 110. Thus, only a single small orifice 150 has been used in prior art pellicle-reticle systems.
The wavelength of the light affects the size of the circuitry that can be produced by photolithography. Shorter light wavelengths allow circuits with smaller features to be produced. Likewise, single-frequency light allows smaller circuitry to be produced than multi-frequency light. Ultraviolet light has traditionally been used. Common wavelengths are 436 nm (called G-Line), 405 nm (H-line), 365 nm (I-line) and 248 nm (called Deep UV), and 193 nm. Prior art photolithography methods and apparatus based on these frequencies have been conducted in ambient temperature, humid air, since nitrogen (N2), oxygen (O2), water (H2O), and carbon dioxide (CO2) do not appreciably absorb ultraviolet light at these frequencies.
Shorter-wavelength ultraviolet light, such as 157 nm, may be used to produce even smaller circuitry features. However, oxygen and carbon containing species present in normal atmospheric air, such as oxygen (O2), water vapor (H2O), and carbon dioxide (CO2), absorb 157 nm ultraviolet light. This may cause irregularities and imperfections in the circuits produced. Unfortunately, due to the single, small orifice 150 and the prior art motivation to minimize airflow into and out of the enclosure, prior art photolithography methods and apparatus are not satisfactory for removal of these species.